This invention relates to a method of treatment for the separation of oil from an oil-containing slurry waste such as, for example, ordinary wastes such as sewage sludges, human and animal excrements, and household wastes and industrial organic effluents such as effluents from food processing plants and effluents from wool scouring plants. The term "oil" as used in this specification shall mean oils and fats, mineral oils, and compounds thereof.
Heretofore, such a slurry waste as mentioned above has been treated by first being dehydrated to some extent and subsequently disposed of in a reclaimed land or burnt in an incinerator. Disposal in the reclaimed land has become increasingly more difficult from the standpoint of the preservation of a healthy environment while disposal in the incinerator has entailed consumption of huge energy. These conventional methods of treatment for the disposal of a slurry waste, therefore, have reached the limit.
To cope with the difficulties, it has been proposed to ferment slurry wastes thereby converting them into fertilizers and other articles of financial worth or at least into articles which can be discarded without entailing the problem of environmental pollution.
Unfortunately, the oil contained in various forms in the slurry waste renders such fermentative treatments extremely difficult. To be specific, the oil contained in the slurry waste notably impairs the efficiency of the removal of water from the slurry waste. Unless a dehydration assistant or moisture content regulator is added in a large amount to the slurry waste being treated, the water content of the slurry waste cannot be sufficiently lowered to the level befitting the fermentative treatment. Moreover, the oil directly obstructs the activities of microorganisms used for the fermentative treatment and consequently renders the fermentation difficult. Incidentally, the sewage sludges of the closed-flow system contain 5 to 20 percent oil, those of the combined-flow system 10 to 15 percent oil, and the household wastes as much as 45 percent or thereabout oil. The effluents from wool scouring plants contain about 40 percent oil and the human excrements about 25 percent oil.
The disposal of such a slurry waste as described above is generally accomplished by at least partly separating the suspended solid particles from the slurry waste through sedimentation and subjecting the resultant supernatant to an activated sludge method or to a bio-oxidation treatment. These treatments, however, are adversely affected to a great extent by the oil contained in the slurry waste. In the case of the activated sludge method, for example, the oil in the slurry waste is adsorbed by and deposited in the form of a coat on the active floc which is the vital agent for the treatment. The deposited coat of oil degrades the active floc's adsorbing capacity, retards the decomposition of waste by the activated sludge, and consequently impairs the treatment of the slurry waste as a whole. In the case of the bio-oxidation treatment, the oil in the slurry waste adheres to the microorganisms responsible for the bio-oxidation and intercepts the respiration of the microorganisms to deprive them of activity and entail the phenomenon of bulking. Consequently, the bio-oxidation is retarded and the treatment is impaired as a whole. In the other methods of treatment, the oil in the slurry waste adheres to the mechanical parts and vessels and consequently causes mechanical troubles and impairs operational efficiency.
The adverse effects which the oil in slurry waste produces in the treatment of a slurry waste as described above could be fully eliminated if the oil contained in the slurry waste was made to sediment amply in conjunction with the suspended solid particles during the sedimentation of these solid particles. When this combined sedimentation is actually carried out, however, the total amount of oil to be separated from the slurry waste increases and the disposal of the slurry waste becomes all the more difficult. Consequently, the improved disposal of slurry waste cannot be attained as desired. The separation of oil from the slurry waste does not merely facilitate the disposal of the slurry waste itself but also counts for much from the standpoint of the efficiency of sewage disposal.
The treatment for the separation of oil from the slurry waste entails the following problems. The first problem is how the oil mingling in various forms in the slurry waste together with large amounts of water and impurities is to be isolated. If the oil in the slurry waste is isolated and recovered in a fairly high yield from the slurry waste, the second problem is how the isolated oil entraining large amounts of water and impurities is to be effectively disposed of. Because these problems remain unsolved, efforts are made to minimize the amount of oil which mixes with sewage sludges by encouraging observance of regulations on the release of wastes. According to the regulations, any waste containing a large amount of oil is not allowed to be released into the sewage system until it is deprived of oil suspended and accumulated in a separate oil layer and subsequently diluted with water.